Party winning masses over has source of power (6)
I believe the answer is:
dynamo
This clue could be a double definition.
'party winning masses over' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'source of power' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'has' is the link.
Can you help me to learn more?
(Other definitions for dynamo that I've seen before include "Electric current generator" , "Generator (sometimes 'human'?)" , "Juice supplier" , "Monday (anag)" , "something electrifying" .)